2 1. Bacillus thuringiensis and Crystal Toxins
Among the most widely used and commercially successful biopesticides are those which employ the microorganism Bacillus thuringiensis.
Bacillus thuringiensis is a widely distributed, rod shaped, aerobic and spore forming microorganism. During its sporulation cycle B. thuringiensis synthesizes proteins that aggregate to form parasporal crystals. The pathogenicity of B. thuringiensis to a variety of sensitive insects, such as larvae Lepidoptera (caterpillars), larvae Coleoptera (grubs) and larvae Diptera (mosquito larvae), is essentially due to these parasporal crystals, which may represent over 20% of the dry weight of the B. thuringiensis cell at the time of sporulation.
It has been reported that different varieties of B. thuringiensis produce serologically different parasporal crystals. Many varieties of B. thuringiensis produce a bipyramidal crystal composed of one or more closely related approximately 130-kDa proteins (P1 proteins or delta endotoxins) that are lepidopteran toxic. Several varieties also produce a flat, cuboidal crystal composed of a 66-kDa protein (P2 protein) that is both lepidopteran and dipteran toxic. B. thuringiensis var. israelensis produces an irregularly shaped parasporal crystal that is composed of three major proteins of approximately 130, 67 and 28 kDa. As in all cases, the size of B. thuringiensis protein toxins are only estimations based on the rates of migration of the proteins during electrophoresis through polyacrylamide gels. The var. israelensis crystal is toxic to dipteran larvae although there are conflicting data as to the toxic activity of the individual proteins that comprise the crystal. Other B. thuringiensis varieties have been discovered which produce a rhomboidal protein crystal that is toxic to coleopteran (beetle) insects.
In this disclosure, the terms "protein endotoxin", "delta endotoxin", "endotoxin" and "toxin" are used synonomously to refer to the crystal protein endotoxins produced by Bacillus thuringiensis insecticidal protein toxin genes, and are not intended to be limited to endotoxins produced by any specific Bt strain.